HVAC Control in the New Millennium.pdf - HVAC.Amickracing

HVAC Control in the New Millenniumthe corresponding wet-bulb and the wet-bulb peak with its correspondingdry-bulb. This will show the local coincident data.As more complicated and sophisticated applications are utilized tomeet energy-saving demands, design maximums based on peak valueswill not suffice. Off-peak values need to be considered. Some types ofdays that must be accommodated are more frequent in occurrence thanmaximum or minimum design days. Examples of these include cloudywith small temperature change, warm a.m., cool p.m. and windy.Machine computations allow rapid calculations of dry-bulb temperaturewith its corresponding wet-bulb maximum for several hoursdaily on both a room-by-room and a zone or building basis. This canresult in more accurate control as well as more compatible system operation.It can also provide energy-consumption estimates.Humidity ControlsHumidity control involves controlling the amount of water vaporpresent in the air in the space. When the relative humidity at the desiredtemperature setpoint is too high, dehumidification is used to reduce theamount of water vapor in the air. When the relative humidity at thedesired temperature setpoint is too low, humidification is used to increasethe amount of water vapor in the air.The relative humidity depends on the dry-bulb temperature. Drybulb temperature and relative humidity are usually specified concurrently.A 70°F dry bulb and 50% RH measured in a room has an actualmoisture content or specific humidity of 54.5 grains of water per poundof dry air. A room with the same specific humidity at 60°F will haveabout 70% RH. At 80°F this becomes about 35% RH.Humidity controllers are also known as humidistats. These unitsare usually contiguous to the thermostat so the humidity is based on theambient temperature. The humidity controller is set at the relative humiditysetpoint. A change in relative humidity from the setpoint causesa control signal to be sent to the controlled component.Humidifiers use water spray, steam grid and steam pan techniques.In a water spray system, the controlled component is a watervalve.If a duct-mounted steam grid humidifier is used, the control signalopens the steam valve at the inlet to the humidifier unit. As the steam©2001 by The Fairmont Press, Inc. All rights reserved.